IAU COMMISSION 14: ATOMIC AND MOLECULAR DATA (DONNEES ATOMIQUES ET MOLECULAIRES) TRIENNIAL REPORT (OCTOBER 1999)

WORKING GROUP 5: MOLECULAR STRUCTURE AND TRANSITION DATA

The following summary has been prepared by E.F. van Dishoeck and J.H. Black, with contributions sent by S. Leach, T. Oka, F. Rostas, L. Rothman, P.L. Smith and K. Yoshino in the summer of 1999. Other topics have been added through a search of the (vast) literature. Because of space limitations, a complete overview or list of references is not possible and only a few highlights limited to gas-phase molecules of actual or potential astrophysical interest are given. With the advent of electronic versions of the astrophysical and chemical physics journals, other related references can readily be found by searching on the appropriate keywords or authors.
 

5.1. Fundamental Data and Databases

The recommended values of fundamental physical constants have been officially revised by CODATA for the first time since 1986. The new 1998 recommended values are based on all data available through 1998 December 31 and will be published in detail soon (Mohr & Taylor 2000). The new values of constants are available electronically through the US National Institute of Standards and Technology (NIST) at

The WWW is rapidly growing into the primary tool to access and search molecular data bases. For example, the GEISA databank of infrared transitions of molecules of atmospheric interest, 1997 edition, is available at

The UMIST data file of rate coefficients for interstellar chemical reactions can be found at

Optical constants of solid materials of interest in astronomy are available through

Infrared spectral atlases of the Sun have been published (Wallace et al. 1996), which are of value for spectroscopic studies of highly excited states of some molecules.

Links to many of the electronic molecular databases can be found through the IAU Astrochemistry working group at

5.2. Electronic Spectra

Small molecules:

The hydrogen molecule is still intensively studied. Photoionization cross sections of He and H₂ have been computed (Yan, Sadegpour & Dalgarno 1998). The UV emission continuum in H₂ excited by electron impact has been investigated (Abgrall et al. 1997). Laser spectroscopy has been used to probe the 86 - 90 nm spectrum of H₂ at high resolution (Reinhold, Hogervorst & Ubachs 1996) and to study some of its inter-Rydberg transitions (Ubachs, Hinnen & Reinhold 1997). The ground state of the hydrogen molecule changes character in strong magnetic fields (Kravchenko & Liberman 1998), which might affect the spectrum of a magnetic white dwarf (see also Detmer et al. 1997, 1998).

Work has continued on the UV spectrum and oscillator strengths of CO. New oscillator strengths have been measured for the B - X (0,0) and (1,0) bands (Stark et al. 1999a) and in the A - X system (Federman et al. 1997). Measurements in this system have been extended from v'=9-17 (Jolly et al. 1997) to v'=13-21 for v''=0 (Stark et al. 1998), and v'=11-23 (Eidelsberg et al. 1999). There are also new ab initio calculations of the A - X transition dipole moment (Spielfiedel et al. 1999), which agree well with these and other experiments. Laboratory data and improved calculations of the intersystem transition oscillator strengths have been obtained (Rostas et al. 1999), which resolve earlier discrepancies. Tunable picosecond lasers have been used to measure lifetimes in the E¹Pi v=0 and v=1 states of three isotopic varieties of CO (Cacciani et al. 1998). High-resolution spectroscopy has been performed on the A - X system in ¹²C¹⁸O (Beaty et al. 1997). The EUV spectrum of CO excited by electron impact has been studied at high resolution (Ciocca, Kanik & Ajello 1997), as well as the A - X system (Beegle et al. 1999). The nd triplet Rydberg series of CO has been studied by laser-induced fluorescence; the resulting analysis led to rotational term values and an improved value of the ionization limit (Mellinger, Vidal & Jungen 1996).

High-resolution vacuum UV spectroscopy has been performed on N₂ (Roncin, Subtil & Launay 1998) and an atlas of the spectrum has been published in the astrophysical literature (Roncin & Launay 1998). An on-line atlas of the spectrum of N₂ has been introduced at the CfA at

Laboratory measurements of the UV absorption of O₂ have continued. The Schumann-Runge bands have been measured in absorption at 670 K and new spectroscopic constants have been derived for the ground state X³Sigma_g^- (Cheung et al. 1996). A comparative high-resolution study of predissociation linewidths in the Schumann-Runge bands has been carried out (Dooley et al. 1998). Oscillator strengths in the Herzberg I system (Yoshino, Huestis & Nicholls 1998) and in the Herzberg II system (Yoshino et al. 1999) have been presented.

Absorption cross sections of H₂O at 120 to 188 nm have been measured by Yoshino et al. (1996, 1997), and the product distribution after photodissociation at 130 nm by Zanganeh et al. (1999). Calculations have been performed by van Harrevelt & van Hemert (1999). Nitrogen oxides have been investigated: absorption cross sections of NO₂ in the visible and UV have been recorded (Yoshino, Esmond & Parkinson 1997), the beta(9,0) band of NO has been investigated in the vacuum UV (Yoshino et al. 1998), and the gamma-band system of NO has been studied (Danielak et al. 1997).

The A³Phi  - X³Delta system (gamma bands) of TiO has been measured in the laboratory and sunspots, providing improved molecular constants, RKR potential energy curves, and Franck-Condon factors (Ram et al. 1999). Several new electronic states of TiO have been identified (Barnes, Merer & Metha 1997). Theoretical transition moments in all the low-lying singlet and triplet systems of TiO have been computed (Langhoff 1997). The absorption spectrum of supersonically cooled CH₃OH has been observed (Sominska & Gedanken 1996). The X²Sigma⁺ and A²Pi states of SiO⁺ have been investigated through fast-ion-beam laser spectroscopy (Rosner et al. 1998). The emission spectrum of the A-X system of SiH and SiD has been recorded (Ram, Engleman & Bernath 1998). Although He₂ has a repulsive ground electronic state, it does possess bound excited states and discrete transitions between them: Focsa, Bernath, & Colin (1998) have obtained new spectra and carried out a global analysis of the six lowest excited states of He₂.

Recent studies of photodissociation processes include SiH⁺ (Stancil et al. 1997), SiO (Jolicard et al. 1997, Drira et al. 1998), CO (Andic et al. 1999), and CH₂ (Kroes et al. 1997).

The upper state of the 141.5 nm transition of CH₂ has been characterized through ab initio calculations by Yamaguchi & Schaefer (1997). Several bands of CH, CH⁺ and their isotopomers have been analyzed (Bembenek 1997a,b; Bembenek, Kepa, & Rytel 1997, Zachwieja 1997, Kepa et al. 1996).

Mahon et al. (1997) have determined transition probabilities in the A¹Pi - X¹Sigma⁺ system of CS. The CS₂ spectrum has been studied by Cossard-Magos et al. (1996, 1997, 1998). New bands in the A¹Pi_u - X¹Sigma⁺_g system of C₃ have been observed through laser induced fluorescence (Baker et al. 1997). A 630 nm band system of FeH has been identified with the e⁶Pi - c⁶Sigma⁺ transition (Goodridge, Hullah & Brown 1998) and the 532 nm green bands with e⁶Pi - ⁶Delta (Goodridge et al. 1997). Laser-induced fluorescence has been measured in ¹³CO₂⁺ Atilde²Pi_u - Xtilde²Pi_g (Varfalvy, Lafleur & Larzillière 1996).

Absorption by SO₂ is important in the atmospheres of Io and Venus; the UV photoabsorption cross sections of SO₂ at 198 - 220 nm have been recorded with a vacuum ultraviolet Fourier transform spectrometer (Stark et al. 1999b).

Large molecules, PAHs, carbon chains and fullerenes:

Considerable progress has been made recently in the investigation of large molecules in the gas phase. Only a small sample of such work can be cited. An exciting development has been the possible identification of some diffuse interstellar bands (DIBs) with an electronic transition in C₇^- based upon laboratory spectra of gas-phase carbon-chain molecules (Tulej et al. 1998, Kirkwood et al. 1998). Extensive studies of the electronic spectra of carbon-chain molecules, radicals and ions have been conducted by Maier (1997) and co-workers (e.g., Wyss, Grutter & Maier 1999, Grutter, Wyss & Maier 1999), using mass selected matrix spectroscopy, supersonic slit jet plasma spectroscopy, hollow cathode spectroscopy and two color photodetachment spectroscopy. More observational data on DIBs are needed to confirm or refute the possible assigment of C₇^-; see, e.g., Cami et al. (1997) and McCall et al. (1999) for recent data.

A summary of the laboratory results on the photophysics of PAHs and the astrophysical implications was given by Leach (1996a). A data base of spectra of neutral and ionized PAHs has been prepared by the NASA-Ames group (e.g., Allamandola, Hudgins & Sandford 1999, Hudgins & Allamandola 1999). The photoionization quantum yield of PAHs is an important parameter in astrophysical modeling of their formation and destruction, and has been measured for a series of PAHs in the 5 - 25 eV range by Jochims, Baumgärtel & Leach (1996), who also established 'rules of thump' for other species. Another important parameter is the structure-dependent photostability of PAH cations, studied by Allain et al. (1996a,b), Allain & Leach (1997) and Jochims, Baumgärtel & Leach (1999). Work has also been carried out on the dissociative ionization of PAHs in the 8 - 35 eV photon energy range (Jochims et al. 1997). Ultraviolet pumping of PAHs has been investigated (Robinson, Beegle & Wdowiak 1997), and electronic spectra of cold gas-phase PAH cations have been recorded (Bréchignac & Pino 1999). Theoretical modeling of the formation and photodestruction of PAHs was performed using the new laboratory data (e.g., Allain et al. 1996a,b; 1997).

The laboratory work on fullerenes concerns ground state and triplet state absorption of neutral species, partially hydrogenated fullerenes and other fullerenes derivatives as well as studies on their non-linear optical properties. The spectra in the range 200 - 800 nm by Bensasson et al. (1997, 1998a,b) and Bini et al. (1998) provided the possibility of testing whether some of the DIBs and other aspects of interstellar extinction such as the 217 nm peak are due to these species. In particular, the spectra of C₆₀H₁₈ and C₆₀H₃₆ do not support the suggestion that the 217 nm peak is due to partially hydrogenated fullerenes. New evidence for the presence of interstellar C₆₀⁺ has been provided by Foing & Ehrenfreund (1997). A special issue of Journal of Physics B was devoted to Fullerenes and it includes articles on the electronic spectra of these molecules (see Leach 1996b, and other articles in that volume).
 

5.3. Vibrational transitions

H₃⁺, long assumed to be the universal protonator and the initiator of interstellar chemistry, has been detected through its vibrational transitions in the molecular material toward the deeply embdeed young stellar objects GL 2136 and W~33A (Geballe & Oka 1996). Subsequently, it has been observed in several other dense clouds (McCall et al. 1999). The agreement between the observed and model column densities provides direct support of the ion-molecule reaction scheme for the formation of interstellar molecules. Subsequent observations of H₃⁺ in the diffuse interstellar medium show surprisingly high abundances, however, which cannot readily be explained by existing diffuse cloud models if the commonly accepted values for the cosmic ray ionization rate and the H₃⁺ dissociative recombination rate are used (McCall et al. 1998a,b; Geballe et al. 1999)

Infrared transition probabilities and the corresponding opacity of SiO have been calculated by Drira et al. (1997) and Aringer et al. (1997). Hot bands in the infrared spectrum of H₂O have been calculated by Viti et al. (1997).
 

5.4. Rotational transitions

For nearly 20 years, the cyanopolyyne HC₉N has been the largest molecule identified in interstellar space. This has changed with the radioastronomical detection of HC₁₁N by Bell et al. (1997). The CfA group led by P. Thaddeus has been very productive in the last three years in finding new spectra of carbon-chain molecules both in the laboratory (e.g., McCarthy et al. 1997, 1998, 1999) and in space. New laboratory spectra include those of HC_n (n=7,8,9,11), HC_2n+1N (n=5,6,7,8), carbon ring chains H₂C_2n+1 (n=2,3,4) and H₂C_2nN (n=2,3), SiC_n (n=3,5,6,7,8) and many more. The species H₂C₅, HC₁₁N, HC_n (n=7,8) have been detected in TMC - 1 (Langer et al. 1997, Bell et al. 1999) and SiC₃ in IRC+10216 (Apponi et al. 1999). The discovery of SiC₃ is particularly noteworthy because this is the first time that the rhombic structure, theoretically predicted to be more stable than the linear structure, is observed in isolated form.

The submillimeter spectra of so-called 'hot cores' in massive star-forming reveal the presence of complex saturated organic molecules at high abundances (e.g., Nummelin et al. 1998). A fraction of the lines is still unidentified, but are likely due to highly-excited rotational lines in the ground- and/or excited vibrational states of known molecules or their isotopes. De Lucia, Herbst and co-workers have provided new laboratory measurements of HCOOCH₃ (Oesterling et al. 1999), CH₃SH (Bettens et al. 1999), CH₃OCH₃ (Groner et al. 1998), C₂H₅OH (Pearson et al. 1997), and c-C₂H₄O (Pan et al. 1998) to address this problem. Updated tables of CH₃OH and ¹³CH₃OH are provided by Xu & Lovas (1997).

The rotational spectra of a wide variety of molecules containing metals and/or second-row elements have been measured in the group of L. Ziurys, and astronomical searches for several of these species have been pursued. Recent examples of laboratory data include those of NaCH (Xin & Ziurys 1998), NaS (Li & Ziurys 1997), AlNC (Robinson, Apponi & Ziurys 1997), FeC (Allen et al. 1996), and alkaline earth hydroxide radicals (Ziurys et al. 1996).

Terahertz spectroscopy of astrophysically relevant species in preparation of the SOFIA and FIRST missions is carried out in the Cologne group of G. Winnewisser (1997). Recent measurements include NH (Klaus, Takano & Winnewisser 1997), NH₂ (Muller et al. 1999), PH (Klisch et al. 1998) and SH (Klisch et al. 1996). Improved determinations of the CO rotational transitions have been obtained for use as secondary standards near 60 THz (Wappelhorst et al. 1997).
 

References